The present invention is directed to a control system and method for internal combustion engines, and more particularly, concerns a throttle position control scheme for electronic throttle control-equipped vehicles.
Electronic airflow control systems such as electronic throttle control systems, replace traditional mechanical throttle cable systems with an xe2x80x9celectronic linkagexe2x80x9d provided by sensors and actuators in communication with an electronic controller. This increases the control authority of the electronic controller and allows the airflow and/or fuel flow to be controlled independently of the accelerator pedal position. Electronic throttle control systems include mechanisms for positioning the throttle plate in response to the driver demand and other vehicle system constraints such as a traction control system.
The most common positioning mechanism is a positioning motor. A closed-loop feedback position controller typically responds to a discrete throttle position value and commanded throttle position. Because the feedback signal is an analog signal that has been discretized by an analog-to-digital converter, its resolution is quantized and may not precisely correspond to a commanded steady-state throttle position. Thus, there is a need for an improved throttle position control system and method.
Electronic Throttle Control (ETC) sets the airflow rate into the engine during idle speed control by controlling the throttle to a precise angle. The vehicle manufacturer would like as fine of positional resolution as possible from the ETC system because it provides fine airflow rate control enabling the manufacturer to markedly improve idle speed control. Classic methods to achieve this goal are costly (e.g. 12 bit A to D, progressive throttle bore). The ETC according to the present invention solves the problem within the micro-controller itself thus yielding a software-only (no variable cost) solution. By forcing the controller into a very specific limit cycle pattern, it can be made to achieve an average position that is of a higher resolution than if it were not to fluctuate. Its limit cycle frequency is high enough to where the fluctuation does not degrade airflow rate control. It in fact improves resolution. Typical systems have {fraction (1/9)} or near {fraction (1/10)} degree resolution. With the system according to the present invention the resolution is improved to {fraction (1/18)} or nearly {fraction (1/20)} degree resolution. In a system that has a natural resolution of {fraction (1/16)} degree, the resolution is improved to {fraction (1/32)} degree.
The system according to the present invention is a feedback position control system. Feedback is provided by a potentiometer-type throttle position sensor. Via circuitry, the sensor inputs a ratiometric voltage at the micro-controller""s analog-to-digital (A to D) input. The controller reads this feedback sensor output as A to D counts (0 to 1023 in this case). Each one of those counts corresponds to a voltage range. If the A to D""s reference voltage is 5.120 volts, each voltage range is nominally 0.005 volts. Each of these voltage ranges corresponds to an angle range. Using a throttle position sensor with an output gain of +16 counts per degree, each A to D count corresponds to a small band of throttle angles that is {fraction (1/16)} degree wide.
If the controller is controlling to a steady A to D count value, the actual position is wandering around in that {fraction (1/16)} degree range. The system according to the present invention eliminates this wandering problem (i.e. uncertainty in actual position) and others by using a limit cycle to force the actual throttle position to continually cross an A to D boundary. For example, the controller carefully quantizes the setpoint value to be xc2xd counts (e.g. xe2x88x92xc2xd, +xc2xd,+1xc2xd, +2xc2xd, +3xc2xd, . . . ). In this way the actual position continually crosses the A to D boundary in a limit cycle and achieves a very repeatable position.
The system according to the present invention preserves all the advantages of the above-described system and adds another feature. That feature is the ability to increase the resolution by a factor of two such that the previous resolution of {fraction (1/16)} degree is improved to {fraction (1/32)} degree.